Surging and temp problems

I have a 1998 SL2 I am having a problem with the temp sensor (s) I am having the problem of surging while idle and while driving. I can literally watch the RPMs rise and temp gauge go crazy up and down. The temp gauge never gets over half way. I have replaced the sensor on the side of block twice (told first one was defective) that fixed it for a while then same thing starts again. Is there anything else that I should check or replace to fix this problem?

check the wire that plugs into the sensor for corrosion. we usually replace the sensor and connector for that problem

yes check the plug, typical for corrosion. but too where did you buy the sensor and does it have a brass tip?